Quick question regarding correct wall construction

[markb]

We are currently drawing up wall plans to give to our builder on tuesday, and I need some advice on the best wall plans to use. In order to seperate the control room from the two studio rooms, and the two studios, what would be the best way of building the walls? I was thinking of having something like the attached picture.

The rooms need isolation from drums and screaming guitar amps, but cost and space are factors that we would like to optimize. My thinking was that by turning the stud on the control room side 'inside-out' we could use the bare rockwool as our room treatment also and cover that with cloth. Would this alter the isolation of our walls though?

Thanks a lot

Al

[sharward]

The problem with "inside-out walls" is that you're effectively reducing your overall air space between the walls. When you're faced with isolating "drums and screaming guitar amps" -- especially the drums though -- you want the widest air gap you can afford.

How much is enough? From what Steve (knightfly) told me some time ago in a telephone consultation, 12" is kind of a "sweet spot" in terms of getting "bang for your buck." Yes, having more than a 12" gap is even better -- but there's a diminishing return. So if you're using 3 1/2" studs, having 5" between frames will give you the "ideal" 12 inches.

Yes, you'd then need to add treatments.

If you're set on doing an inside-out wall, then bump the distance between frames by an other 3 1/2" to 4 1/2" or so.

[markb]

CHeers for the reply.

If you look at my drawing (which I have just noticed) the distance between the two leaves (going from plasterboard to plasterboard) is actually 12" - would this then be sufficient?

Lets say we did it the traditional method with the plasterboard layers on the outside of each leaf, keeping the 12" air gap - would their be any difference in the TL of walls like this, compared to mine?

Al

[sharward]

I'm not the math whiz, nor do I have the fancy calculator tools that some folks here have... But I would think you should be OK with that plan.

Don't take my word alone though! I'm much better spotting things wrong with certain types of plans than I am endorsing them!

--Keith

[markb]

Thanks a lot for your input mate - much appreciated!

Anyone else like to check our ideas?

Al

[knightfly]

Two layers of 5/8 gypsum each side of a double framed wall with 8" between masses vs. 12" between masses - both calculate at STC 67 (centered at 500 hZ) - the 8" gap gives 35 dB TL @ 50 hZ, while the 12" gap gives 38 dB TL @ 50 hZ. It's irrelevant which side the insulation goes on, as long as it's in good contact with the gypsum in each frame - main purpose is damping of the gypsum layers.

But, in order to maintain the same effective air gap (actual distance from one pair of gypsum panels to the opposing pair) you WOULD need to move your second frame, since any insulation BETWEEN the panels is considered as "air".

Before you decide 3 dB isn't worth the loss in usable space, this is the frequency that will come through the wall. This means that 3 dB @ 50 hZ will make the WHOLE WALL sound 3 dB quieter, which can sometimes be enough difference to eliminate problems with neighbors.

If you want even better, with more expense but NOT more loss in floor space, look at using Green Glue between the layers.

Also, it's been found that using ALL 5/8" works as well or better than differing thickness; this is because (1) the LF is where it's hardest to isolate, and MASS is what you need there, and (2) it's too easy to grab the wrong thickness when there's both kinds on the job site.

If you can spend a bit more, THREE layers of 5/8 on either side of a double frame with only 8" air space comes out to STC 75, with TL @ 50 hZ of 41 dB. That's without GG, and total wall thickness would be 11-3/4" as compared to 14-1/2" for the double 5/8 and 12" air gap.

BTW, 41 dB @ 50 hZ translates to being able to play drums at around 96-98 dB before they become audible on the other side of the wall, since human hearing @ 50 hZ has a threshold of about 55 dB... Steve

[markb]

Thanks for the reply, thats some interesting info there.

I think my diagram is a little unclear - where I have drawn the 8" airgap, that DOESN'T include the insulation within the studio-side studding. So if you take the distance from the gypsum layer on the studio side, you have the 4" of rockwool within the frame, THEN an 8" empty airgap, then the gypsum panel, with the control room rockwool/stud on the outer side.

Will this not then be adequate, since I am having the optimum 12" airgap, my gypsum layers are damped directly by insulation, and I'll have double (perhaps even triple) layers of gypsum? Or am I misunderstanding entirely?

Thanks a lot, great advice so far.

Al

[sharward]

I think the way you've drawn it, and what you have described, will be fine.

My only issue is use of the word "optimum." Technically, 12" of air gap isn't optimum. Miles is optimum. But, yes, 12" is a healthy balance, since air gaps aren't free (meaning, you lose square footage if you increase the gap).

It is interesting how two different wall constructions (in this case, a difference only of the air gap) can have the same STC rating, yet be markedly different in terms of low end isolation. I read in a few places that a 1db reduction is barely noticable to the human ear, 2db is noticable, and 3db is significant.

--Keith

[knightfly]

You've got it - and if you use THREE layers of 5/8 gypsum each side with your 12" gap, the numbers come in at STC 75, with 45 dB @ 50 hZ.

Changing one side to 4 layers of 1/2" would improve STC 2 dB (mostly by lessening the coincidence dip due to differing thicknesses of gypsum) and add another dB (46) @ 50 hZ, due mainly to the slightly increased overall mass... Steve